A conversion known as direct lithographic or "di-litho" simply replaces the letterpress plate and impression cylinders with offset plate and blanket cylinders. In the di-litho process, the web is fed between the dilitho plate and blanket cylinders so that the process is not a true offset printing process.
In accordance with an invention of the present inventor and Duane H. Houy, which is the subject matter of U.S. patent application Ser. No. 32,240, filed Apr. 20, 1979, and assigned to the same assignee as the present invention, a method for converting a web fed letterpress unit having a main frame, a pair of plate cylinders laterally spaced in the frame for mounting printing plates thereon, means for applying films of ink to the plates, and a pair of impression cylinders in respective rolling contact with the plates on the plate cylinder, into a web fed offset printing press unit includes removing the pair of letterpress plate cylinders and the pair of letterpress impression cylinders. Internal support means for rotatably supporting offset cylinders in the offset printing configuration with a bearing internal to the main frame is affixed to the main frame. One embodiment of the internal support means includes an auxiliary frame. A lower portion of an auxiliary frame for receiving offset cylinders in an offset printing press configuration is placed inside the main frame. A pair of offset plate cylinders is then installed. The offset plate cylinders might fit into the bores and bearings for the letterpress plate cylinders in which case they can be installed directly, otherwise one or both of the offset plate cylinders is installed in the auxiliary frame. The lower portion of the auxiliary frame is then installed, using pilots to guide the installation of the auxiliary frame. The pilots, in one embodiment, are affixed to the auxiliary frame and are designed to be inserted into the original cylinder bores of the main frame. After the auxiliary frame is installed, a pair of blanket cylinders is installed in close proximity to a position for respective rolling contact with the plates on the plate cylinder and the other blanket cylinder. In some circumstances the lower portions of the auxiliary frame are completely installed prior to the installation of the plate cylinders.
It is common for letterpress units to also have a pair of laterally spaced form rollers in respective rolling contact with the plates on the plate cylinder. A preferred conversion according to that method includes removing at least one pair of form rollers, and mounting micrometric adjusting sockets for receiving the form rollers for respective rolling contact between at least one pair of form rollers with the plates of the plate cylinder after the pair of offset plate cylinders is installed. It is also preferred to plate the ink drums with copper, and add dampener motions.
The gear train used to drive the letterpress unit cylinders is not suitable for driving the offset cylinders. A new offset gear train must, therefore, be installed for imparting motion from one of the offset press unit cylinders to other offset press unit cylinders. Some of the gears for the letterpress unit can be removed, but many will still be used for driving at least one of the cylinders and for driving the inking drums. In converting some letterpress units, it will be possible to extend the shaft of the offset cylinders through the bores for the letterpress unit cylinders in the main frame by removing the bearings for the letterpress cylinders. In such a case, the offset gear train can normally be installed on the outside of the main frame to allow more room inside the frames for wider web widths. With the cylinder shafts supported by the auxiliary frame inside the main frame, unwanted vibrations are caused by supporting the weight of the gears on the outside of the main frame. Otherwise, the offset gear train can be installed inside of the main frame, but installation of the gear train inside the main frame makes it difficult to access the gears for maintenance and impossible to remove the gears without removing the upper portions of the auxiliary frame and the cylinders.
Considering the cylinders to include both the drum body and the shaft, it is frequently necessary to make the drum bodies for the offset press shorter than the drum bodies were for the letterpress in order to allow additional room for the auxiliary frame and the offset gear train when it is inside the main frame. This shortening of the drum bodies narrows the allowable web width, but this normally does not present a problem in the United States since the industry has fairly well settled on a 58 inch web width which is sufficiently narrower than the typical 68 inch web width for which most letterpress units were designed. This does, however, present a problem in Europe and other countries that still use a wider web width. The total length of the cylinders is also shorter when the shafts cannot extend through the main frame bores for the letterpress cylinders.
A preferred conversion according to that method and apparatus includes the installation of means for bodily swinging various cylinders into different positions for different printing arrangements and for throw-off in case of web wrap, for changing plates, or other cylinder maintenance.
A special method according to that invention for converting a letterpress unit that has at least a half deck with a deck plate cylinder and a deck impression cylinder is to remove the deck impression cylinder, install a deck blanket cylinder in a position for rolling contact with the plates of the deck plate cylinder, if necessary replacing the deck plate cylinder with a deck offset plate cylinder, and installing an impression cylinder in the vicinity of the cusp formed by the pair of blanket cylinders for placing in rolling contact with the deck blanket cylinder and for placing in rolling contact with all three cylinders, the deck blanket cylinder and the pair of offset blanket cylinders. This method also includes the situation where there is a full deck, the full deck being simply two half decks.
In a conversion involving at least one half deck, a means is installed for bodily swinging each of the blanket cylinders and the impression cylinder between a first position in which the blanket cylinders make contact with the plates of their respective plate cylinders and with the impression cylinder, a second position in which the pair of blanket cylinders make contact with each other and are isolated from the impression cylinder, and the deck blanket cylinder makes contact with the impression cylinder, and a third position in which the blanket cylinders and the impression cylinder are displaced into a throw-off position in which the blanket cylinders are isolated from their associated cylinders. Such a method can also include installing a means for bodily swinging the remaining offset cylinders wherein all offset cylinders are displaced in the third position.